Settings: Proper priority hierarchy

Remove old defaults system
Introduce priority-based fallback list
Use new functions for map_meta special functions
Change groups to use end tags

Unittest changes:
 * Adapt unittest to the new code
 * Compare Settings objects

src/script/scripting_mainmenu.cpp

@@ -31,7 +31,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 extern "C" {
 #include "lualib.h"
 }
-
+#include "settings.h"
 #define MAINMENU_NUM_ASYNC_THREADS 4